In all vertebrates there are brain nuclei which receive topographically organized projections from the retina. Understanding how such specific neuronal connections develop is critically important in the analysis of nervous system function. The first step in such a study of development of ordered neuronal connections is to establish the pattern of neurogenesis in the connecting structures. We propose to analyze the development of the primary visual projection, using a teleost fish as a model system. These animals have numerous advantages for such a study, primarily the fact that they continue to grow throughout their lifetime, adding neurons to the retina and brain. Moreover, the brain structures responsible for vision are large and easily accessible, making experimental manipulation easier. Preliminary evidence suggests that the retina and the tectum, which receives the majority of the visual fibers, grow in distinctly different patterns. The retina is thought to add concentric rings of new cells and the tectum to grow in a crescent shaped pattern. This suggested difference in cell addition in a sensory organ and its primary projection poses a serious enigma for the maintenance of an ordered connection between the two loci. We propose to follow the development of the primary visual structures from embryogenesis through adulthood using radioactively labeled DNA precursors to identify newly generated neurons. By understanding how cells are added in these structures, we can then predict how the integrity of the retinal projection of the brain is maintained during growth.